Silanes have been grafted to a polymer film to form an oxygen barrier coating through the use of electron beam processing.
U.S. Pat. Nos. 4,803,126 and 5,009,924, specify the need for electron beam irradiation in order to form the silane barrier layer or the film. Since electron beam processing discolors and embrittles paperboard, it is desirable to utilize another method for producing the materials on polymer coated paper or paperboard.
Electron beam irradiation tends to penetrate well beyond the coating into the paper or paperboard embrittlement and discoloration can occur throughout a substrate to the uncoated side.
Previous approaches, prior to electron beam irradiation, to improving barrier properties of, for example polyolefin films, have involved the application of a second polymer layer to the base film, often by extrusion, in which the second polymer layer has substantially superior barrier properties against permeation oxygen. Examples of these materials are polyvinylidene chloride, ethylene vinyl alcohol copolymer, and the like.
The additional expense involved in the use of an additional layer and the difficulty found in adhering the second polymers to the polyolefin film makes this approach somewhat undesirable.
Silane materials have heretofore been used to modify the surface of polymers to improve adhesion. A variety of organofunctional silanes are useful as coupling agents. Glass fiber polymer laminates have been substantially improved by the use of certain silane coatings on the glass fibers which will graft and bond strongly to specific polymers that otherwise do not wet the glass surface well.
Certain organofunctional silane coupling agents will promote adhesion between dissimilar polymers which are otherwise difficult to bond. Bonding of polyethylene to polyester using a hot melt adhesive following the application of the proper coupling agent to each polymer surface is accomplished.
As mentioned already, electron beam irradiation can discolor paperboard/paper, causing a bleached substrate to turn yellow. This result has a negative influence on product appearance and graphic arts quality. Electron beam irradiation can also embrittle paper/paperboard, leading to scoreline cracking or other converting runnability problems. Since the electron beam tends to penetrate well beyond the coating into the paper, embrittlement and discoloration can occur throughout a paper substrate to the uncoated side. Additionally, there has been the need for grafting the silane to a polymer substrate, which limits package construction options.
The present invention utilizes ultraviolet radiation to cure a silane coating and form a barrier to oxygen permeation. The ultraviolet radiation is absorbed almost completely in the coating layer and that portion which reaches the paper has little effect on its strength and cannot discolor the uncoated side of even a thin paper substrate. Further, one can use paper substrates which do not have extrusion coated polymer layers on the surface, thereby increasing the number of construction options.